1. Technical Field
The present invention relates to a cryogenic power generation system employing fuel cells, and particularly to an LNG cryogenic power generation system equipped with a CO.sub.2 separator taking advantages of cryogenic LNG.
2. Background Art
Power generation systems using fuel cells are known in the art and some systems employ molten carbonate fuel cells. A conventional molten carbonate fuel cell generally includes an electrolyte plate (tile) soaked with carbonate, a cathode chamber (oxygen electrode) and an anode chamber (fuel electrode). The electrolyte plate is made from a porous material and the carbonate serves as the electrolyte. The electrolyte plate is sandwiched by the cathode chamber and the anode chamber. Oxidizing gas is introduced to the cathode chamber and fuel gas in introduced to the anode chamber to cause the power generation due to an electrical potential difference between the cathode chamber and the anode chamber. In a conventional power generation system using molten carbonate fuel cells, the above-described fuel cells are generally stacked one after another via separators to define a multi-layer fuel cell unit or a stack of fuel cells.
One example of such power generation systems is illustrated in FIG. 5 of the accompanying drawings. As illustrated in FIG. 5, before air A reaches a cathode chamber 102 of a fuel cell 100 via an air feed line 108, the air A is compressed by a compressor 104, cooled by a cooling device 105, compressed by another compressor 106 and preheated by an air preheater 107. Part of the air A in the air feed line 108 is branched to a reformer 110 by a branch line 109. Gases CG discharged from the cathode chamber 102 (also called "cathode exhaust gas CG") are introduced to a turbine 112 through an exit line 111 and expelled via the air preheater 107. Gases AG discharged from the anode chamber 103 (also called "anode exhaust gas AG") contain H.sub.2 O and CO.sub.2. Thus, moisture H.sub.2 O of the anode exhaust gas AG is removed and the separated moisture H.sub.2 O is recirculated to the system. The anode exhaust gas AG of the fuel cell 100 is cooled by a heat exchanger 113, heat-exchanged with natural gas NG in a preheater 114 and cooled by another cooling device 116. In the cooling device 116, the anode exhaust gas AG is condensed, then introduced to a gas-liquid separator 117 to separate moisture component from gas component. The gas component which contains CO.sub.2 is fed to a combustion chamber of the reformer 110 by a blower 118 through a line 119 extending to the heat exchanger 113. The moisture or water component H.sub.2 O is pressurized by a pump 120 and fed to an evaporator 121. In the evaporator 121, the water H.sub.2 O is heated to steam, then fed to an entrance of the reformer 110 via a superheater 115 through a line 122 such that it is mixed with the natural gas NG. Fuel gas produced in the reformer 110 is introduced to the anode chamber 103 of the fuel cell 100 by a piping 123. Gases discharged from the combustion chamber of the reformer 110, which contain CO.sub.2, are fed to the cathode chamber 102 of the fuel cell 100 through a line 124 together with the air of the line 108. An evaporator 115 is provided between the preheater 114 and the cooling device 116 such that the anode exhaust gas AG flows therethrough. Numeral 101 designates an electrolyte plate and numeral 125 designates a desulfurizer.
In the above-described power generation system using molten carbonate fuel cells, the moisture H.sub.2 O of the anode exhaust gas AG discharged from the anode chamber 103 is removed by the gas-liquid separator 117, and the CO.sub.2 -containing-gases are combusted in the combustion chamber of the reformer 110 before they are fed to the cathode chamber 102. Therefore, a CO.sub.2 separation from the gases and a recovery of CO.sub.2 are not considered. Consequently, the conventional power generation system is not designed to recover CO.sub.2 and CO.sub.2 is expelled to atmosphere.